Uses a conditional move for usage counter increment instead of a branch

runtime/vm/intermediate_language_x64.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/globals.h"  // Needed here to get TARGET_ARCH_X64.
 #if defined(TARGET_ARCH_X64)
 
 #include "vm/intermediate_language.h"
 
 #include "vm/dart_entry.h"
@@ skipping 854 matching lines @@
                                                        bool opt) const {
   const intptr_t kNumInputs = 1;
   return LocationSummary::Make(isolate,
                                kNumInputs,
                                Location::RequiresRegister(),
                                LocationSummary::kNoCall);
 }
 
 
 void LoadClassIdInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
-  Register object = locs()->in(0).reg();
-  Register result = locs()->out(0).reg();
-  __ LoadTaggedClassIdMayBeSmi(result, object);
+  const Register object = locs()->in(0).reg();
+  const Register result = locs()->out(0).reg();
+  Label load, done;
+
+  // We don't use Assembler::LoadTaggedClassIdMayBeSmi() here---which uses
+  // a conditional move instead---because it is slower, probably due to
+  // branch prediction usually working just fine in this case.
+  __ testq(object, Immediate(kSmiTagMask));
+  __ j(NOT_ZERO, &load, Assembler::kNearJump);
+  __ LoadImmediate(result, Immediate(Smi::RawValue(kSmiCid)), PP);
+  __ jmp(&done);
+  __ Bind(&load);
+  __ LoadClassId(result, object);
+  __ SmiTag(result);
+  __ Bind(&done);
 }
 
 
 CompileType LoadIndexedInstr::ComputeType() const {
   switch (class_id_) {
     case kArrayCid:
     case kImmutableArrayCid:
       return CompileType::Dynamic();
 
     case kTypedDataFloat32ArrayCid:
@@ skipping 4974 matching lines @@
   __ movq(R10, Immediate(kInvalidObjectPointer));
   __ movq(RBX, Immediate(kInvalidObjectPointer));
 #endif
 }
 
 }  // namespace dart
 
 #undef __
 
 #endif  // defined TARGET_ARCH_X64